1. Field of the Invention
The present invention relates generally to a display apparatus and a method for reproducing colors therewith, and more particularly, to a display apparatus adapted to detect skin color from (R, G, B) video signals and calculate luminance signal level to correct the luminance signal level of the (R, G, B) video signals and a method for reproducing colors therewith.
2. Description of the Related Art
In a color TV system, it is an object to reproduce images on a display device in such a manner that the brightness of the images is proportional to the luminance of the original scene, which has received the standard lighting, and the colors thereof are the same as in the chromaticity coordinate of the original scene.
In the case of a National Television Standards Committee (NTSC) color TV system, it is requested that ideal imaging characteristics are obtained from a standard camera (means for obtaining images) based on an NTSC standard monitor. A TV set reproduces the same chromaticity coordinate as in the original scene, which has been illuminated by C light source.
However, the phosphorescent substance and the standard white color of most current color TV sets have variously deviated from that of the NTSC standards. This is because the screen was dark due to the low luminous efficiency of the phosphorescent substance, when the NTSC standard specifications were established, and other phosphorescent substances having better luminous efficiency have been used. In addition, the color temperature of the standard white color has been raised for a brighter screen.
For these reasons, TV manufacturers in the USA and Japan chose 9,300K as the standard white color of TV sets in 1958, when P22-series phosphorescent substance was used for the first time, and have produced TV sets which reproduce colors based on that standard white.
Afterwards, it has become a trend in Korean or Japan, wherein orange fluorescent lamps of about 6,000K are widely used for indoor lighting, to arbitrarily choose the standard white color of TV sets from a range between 11,000K and 13,000K, which is higher than 9,300K.
As the standard white color of the transmission side (camera for obtaining video signals) differs from that of the reception side (display), color reproduction distortion occurs naturally.
As TV signal sources, public broadcasting, such as NTSC broadcasting or HDTV broadcasting, and various signal sources having different standards, such as DVD or digital still camera, coexist. Therefore, gamma processing must be performed while considering the TV set signal sources having various formats and standards, when transmitting TV signals, and the display characteristics of the display device (CRT), when making signals of each signal source.
In addition, TV sets must consider the gamma processing of the transmission side when reproducing colors. Otherwise, distortion occurs during color reproduction and the fidelity to the original colors degrades drastically.
Besides the above-mentioned problems, deviation of the phosphorescent substance coordinate of the transmission-side standard monitor from that of the TV sets is another main cause of chromaticity error.
Therefore, original colors can be reproduced with fidelity only when the type of the input signal source and that of the video display device are fully considered together with the camera's gamma characteristics of the transmission side during color reproduction.
A TV system according to the prior art will now be described with reference to the accompanying drawings.
FIG. 1 is a block diagram showing the construction of a TV transmission/reception system to illustrate the construction of a transmission/reception apparatus of a conventional TV system, FIG. 2 shows the trajectories of colors having the same phase on x, y coordinate system, FIG. 3 shows iso-hue and iso-chroma trajectories when the value is 7 in a Munsell system, and FIG. 4 shows iso-hue and iso-chroma trajectories when the value is 9 in a Munsell system.
In a conventional TV system, color signals are processed in the transmission side as follows: a camera lens 10 obtains images from an object illuminated by a light source. The obtained images pass through an optical distributor 20 and red, green, and blue sensors 30, 31, and 32 having an optical filter function to be sensed as red (R), green (G), and blue (B) signals.
The sensed R, G, B signals pass through a gain adjustment unit 40 for proper camera gain adjustment and a gamma correction unit 50 for gamma correction. The R, G, B video signals, after gamma correction, are encoded by an encoder 60 and are transmitted to the reception side via a transmission unit 70.
Color signals are processed in the reception side as follows: signals transmitted via the transmission unit 70 of the transmission side are received and decoded by a receiver/decoder 80. The decoded R, G, B signals pass through a display gain adjustment unit 90 for proper R, B, B gain adjustment and are displayed on a CRT 100 so that the viewer can watch images.
The imaging apparatus of the transmission side or the display apparatus of the reception side can image or reproduce only a limited range of colors, in contrast to natural colors humans can sense, which is referred to as the color reproduction gamut of the apparatus.
If the color reproduction gamut of the imaging apparatus is identical to that of the display apparatus, the display apparatus can reproduce the same colors as imaged by the imaging apparatus but, if different, it cannot reproduce the same colors.
When the color reproduction gamut of the signal source is different from that of the display apparatus, particularly, the display apparatus chooses a suitable color from its own color reproduction gamut, which corresponds to the color from the signal source, and reproduces it. When colors are reproduced in this manner, the colors from the signal source and the reproduced colors appear to be different from each other in the Munsell system which is based on the human vision.
In FIG. 2, a number of colors including “jap. girl”, “white flesh tone”, and “yellow” are shown based on density and luminance while varying their color reproduction gamut. It is clear that the density and luminance of the colors change as their color reproduction gamut varies, even when they have the same phase. In other words, colors having a different color reproduction gamut appear as different colors in the Munsell system, even when they have the same phase.
As shown in x, y coordinate system of FIG. 2, colors having the same phase exhibit different hue characteristics when the chroma level is low, but do not when the chroma level is high.
When the trajectories of colors are drawn based on the color density and luminance, which are perceived to have the same hue by human vision, they appear to have different hues as shown in FIGS. 3 and 4.
Such characteristics can be easily understood from FIGS. 3 and 4 which illustrate iso-hue and iso-chroma trajectories when the value is 7 in a Munsell system and when the value is 9, respectively, as well as from FIG. 2 which illustrates the iso-phase color trajectories in the TV system shown in FIG. 2.
Therefore, when colors having the same phase are reproduced based on luminance according to the prior art, a great difference in hue is noticed by the viewer when the luminance signal level is low, although there is little problem when the luminance signal level is high.